1. Field of the Invention
The present invention relates to an image signal processing device and, more particularly, to an image signal processing device capable of selectively recording image signals in a memory card or similar data recording medium or reproducing them read out of the medium.
2. Description of the Related Art
A still camera has traditionally referred to an apparatus of the kind recording pictures in a medium implemented as a film which carries a layer of silver halide sensitive material on the surface thereof. A recent achievement in the imaging art is an electronic still camera operable with a recording medium in the form of a memory card which is loaded with a semiconductor memory. On shooting a desired object, this type camera records the resulting image signal in the memory card, which is removably mounted to the camera, in a digital signal format. The electronic still camera is advantageous over the traditional still camera in various aspects. For example, the camera has a small size and light weight and is capable of storing image data in a memory card and reproducing them instantly. In addition, the camera extends the applicable range of digitized image data. Moreover, regarding quality, pictures produced by this type of camera are now comparable with pictures produced by silver halide sensitive type of photography.
In a specific form of electronic still camera, an optical image representative of a scene is incident to an imaging device via a lens and converted to an electric image signal thereby. A preprocessing circuit executes various kinds of signal processing, including white balance adjustment and tone correction, with the image signal. An analog-to-digital converter digitizes the processed image signal to produce a corresponding digital image signal, i.e., image data. The image data is written to a frame memory. At the same time, the image data are transformed to image data represented by a luminance signal and by chrominance signals. The image data stored in the frame memory are read out in a plurality of blocks, compressed, or coded, by a compander (compressor/expander), and then written to a memory card which is removably mounted to the camera. The image data written to the memory card are expanded, or decoded, by the compander, once written to the frame memory, and then transferred to a playback circuit to display a still picture on a video monitor.
The image signal processing device built in the above-described type of electronic still camera has some problems left unsolved since the number of pixels is increasing in order to achieve image quality comparable with that of silver halide sensitive type of photography. For example, assume an image signal processing device included in an electronic still camera which uses a solid state imaging device having 1,300,000 pixels and implemented by a CCD (Charge Coupled Device) image sensor. A problem with this kind of image signal processing device is that the number of shots which can be stored in a memory card is reduced due to the huge amount of image data. On the other hand, when the compression ratio of an image is increased in order to increase the number of shots to be written to a memory card or to display an NTSC (National Television System Committee) standard video signal on a monitor, noticeable distortions occur when the compressed image is reproduced. Furthermore, high-speed image processing and data transfer are not achievable with this kind of device unless the device is provided with a master clock w h o se frequency is as high as 50 MHz to 75 MHz. In addition, such a high clock frequency is disadvantageous in that power consumption is aggravated and results in the need for a bulky battery which would degrade the portability of the camera. Further, the increase in power consumption increases the amount of heat generation and, therefore, the size of a heat radiation package built in the camera.